Configure a server side CI gate to test client

Motivation: now it's possible to make changes in Mistral server that are not compatible with the client. The only way to see it now is to submit a patch to the client (or recheck an existing one). If we don't make changes in the client often we may not see issues during long time.

Suggestion: reuse existing functional tests in the client and also run them for every server-side commit in non-voting mode (we can't use voting because it'll be a circular dependency then).